This invention relates to a process for the production of starch based hot melt adhesives. More particularly, this invention involves the formulation of a hot melt adhesive using selected starch ester wetcakes.
Hot melt adhesives are 100% solid materials at room temperature which do not contain or require any volatile solvents. They are solid materials at room temperature, but, on the application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form and gains its cohesive strength. In this regard, hot melt adhesives differ from other types of adhesives which achieve the solid state through evaporation or removal of solvents or by polymerization.
Generally, hot melt adhesives have been based on synthetic and natural resins and waxes, particularly petroleum derived polymers such as polyethylene, ethylene-vinyl acetate, styrenic block copolymers, and polypropylene to name a few. While starches have been used as adhesives in aqueous systems for many years, they have not been used in hot melts as the base or main-functional adhesive material. This is primarily because starch will normally not melt in the absence of water or solvent and it has therefore been difficult to formulate a starch based hot melt composition.
Recently, hot melt adhesives based on intermediate or high DS (degree of substitution) starch esters have been disclosed in copending application Ser. No. 08/106,023 filed on Aug. 13, 1993. In European patent publication 511 916 of Nov. 4, 1992 hydrogenated starch hydrolyzates were shown useful in forming hot melt adhesives.
In formulating hot melts, water is not generally used (typically avoided) and when starches such as the starch esters described above are used, they are dried prior to blending and formulating to remove or reduce the moisture content. This not only involves an extra processing step but additionally the formation of starch powder or dust creates difficult handling problems. It is well documented that starches having under about 15% moisture content pose a serious explosive potential.
Now it has been found that starch based hot melt adhesives can be formulated without a starch ester drying step and other processing precautions normally needed by using selected starch esters in wetcake form as described herein. Therefore, the starch wetcakes of this invention provide easier, safer handling and do not require special precautions.